This invention pertains to stable, preferably cross-linked, lipid bilayer vesicles and to methods of making such vesicles. Lipid bilayer vesicles are of considerable current interest as models for biomembranes, as carriers of drugs, and as devices for photochemical solar energy conversion. These particles are comprised of alternating lipid bilayer and aqueous compartments arranged in a concentric fashion.
Naturally occurring and synthetic phospholipids when dispersed in water form shell-like structures known as liposoomes. As used herein, the term "phospholipid", refers to a class of compounds also known as phosphoglycerides, of which the parent compound is a phosphoric ester of glycerol in which at least one of the other glycerol hydroxyl groups is esterified with a fatty acid. Liposomes are of particular interest because of their close structural relationship to biological membranes. However, in order to maximize their utility for biomechanistic studies (membrane modeling) and for medical applications (drug delivery), liposomes should be prepared under the mildest conditions possible, so that sensitive comembrane and entrapped components can be effectively incorporated.
Liposomes formed from monomeric lipids, such as phosphatidylcholines are of limited practical utility because they are thermodynamically and biologically unstable and the rate of leakage of entrapped drugs from them is difficult to control.
Recently, polymerized vesicles formed from polymerizable materials have been synthesized. Polymerizing the lipid material from which the vesicle is formed is a highly effective method for achieving the desired enhanced stability but these types of vesicles are often difficult to synthesize. Vesicles of this type are disclosed in U.S. Pat. No. 4,594,139 (of common inventorship herewith) in which it was shown that 1,2-bis(11-mercaptoundecanoyl)-sn-glycero-3 phosphocholine can be polymerized in vesicle form via oxidation with hydrogen peroxide and subsequently depolymerized via dithiothreitol to yield the regenerated monomer.
It has also been shown by S. Regen et. al., J. Amer. Chem. Soc. 1985, Vol 107, page 5804, et seq., that a macrocyclic disulfide analog of the above compound can be polymerized in the vesicle state through ring-opening polymerization initiated by a catalytic amount of a reducing agent such as dithiothreitol.
The synthesis methods for both of these polymerized vesicle forms are somewhat elaborate, and require protecting the reactive thiol link throughout the synthesis process.
In both of the above cases, polymerization proceeds to completion within 4 hours at 50.degree. C. affording linear polymers having a maximum number average degree of polymerization of 28. While the polymerization methods referred to above result in improved physical shelf-life of the vesicles, they do not yield vesicles that are crosslinked and which are also highly stable against lysing agents such as sodium dodecylsulfate (SDS).